1. Field of the Invention
The present invention concerns an optical data transmission system.
2. Description of the Prior Art
Transmission of data over long distances, especially transoceanic telephone transmission, is increasingly effected optically using optical fibers. This has significant advantages compared to electrical transmission; in particular losses are low and there is less signal distortion.
In such telecommunication systems, in particular for undersea telecommunications, there is a constant need for higher bit rates without degrading signal quality and whilst remaining within reasonable cost limits.
The data transmitted by optical communication systems is in binary form, i.e. the information is represented by xe2x80x9c0xe2x80x9d bits and xe2x80x9c1xe2x80x9d bits. The shape of the signals transmitted has a decisive effect on the performance of the transmission system, losses and noise depending on the shape of the signals.
Until now, binary digital signals have mainly been transmitted in the NRZ format, i.e. in the form of rectangular signals, xe2x80x9c0xe2x80x9d usually being the low level and xe2x80x9c1xe2x80x9d the high level. For successive xe2x80x9c1xe2x80x9d the signal remains at the high level and does not return to the xe2x80x9c0xe2x80x9d level or low level (hence the name of the format: NRZ=no return to zero).
Formats have been researched recently that enhance communication performance. The format of most interest at present is the soliton format. In this format, the xe2x80x9c1xe2x80x9d are pulses, generally positive (bright soliton) and the xe2x80x9c0xe2x80x9d are the low level. Between two successive xe2x80x9c1xe2x80x9d, i.e. between two soliton pulses, the signal returns to the low level.
The shape of the soliton pulses as a function of time is the inverse of the square of a hyperbolic cosine. The peak power and the mid-amplitude width of the soliton pulse are linked by a precise relationship that depends on the characteristics of the fiber, namely the Kerr index n2 and the dispersion D. The index n2 is the non-linear part of the refractive index, i.e. the part which depends on the power of the optical signal. This relationship between the peak power and the half-amplitude width is such that the effects of dispersion and of non-linearity cancel each other out. As a result, this shape of pulse does not spread as it propagates, despite the inevitable chromatic dispersion of optical fibers.
Nevertheless, optical transmission systems using soliton pulses are affected by noise and this has to be eliminated. The noise comprises fluctuations in the amplitude of the soliton pulses and fluctuations in the temporal position of the pulses. These temporal fluctuations (also known as xe2x80x9cjitterxe2x80x9d) originate in random fluctuations of the central wavelength of the soliton pulses due to non-linear interaction between these pulses and the spontaneous emission noise of optical amplifiers (or repeaters) on the transmission line; this temporal jitter is further influenced by dispersion and increases with the length of the line and with the number of repeaters. A repeater is an optical amplifier designed to provide a particular optical signal power at its output, the amplification compensating the inevitable attenuation of the optical signal transmitted by an optical fiber.
Various solutions to the problem of reducing or eliminating these fluctuations or jitter from soliton pulses are known in themselves.
A first solution consists in providing on the line at least one modulator which repositions the pulse in time using the clock signal extracted from the data. Better results are expected using a phase modulator. Further information on the correction achieved by a phase modulator of this kind can be found, for example, in an article in IEEE Photonics Technology letters, volume 8, number 3, March 1996, pages 455-457 entitled xe2x80x9cGordon-Haus Jitter Suppression Using an Intra-Span Phase Modulator and Post Transmission Dispersion Compensatorxe2x80x9d by N. J. Smith, N. J. Doran and W. Forysiak. The above article also mentions that correction by means of a phase modulator can be complemented by a dispersion compensation component.
This phase modulation techniquexe2x80x94which is an active correction techniquexe2x80x94can be associated with passive compensation using optical filters having a wavelength centered on the wavelength of the line. These filters further stabilize the solitons; the stabilization is only partial, however, since such filters necessarily attenuate the optical signal and therefore require the use of high-gain amplifiers which are costly and can also introduce noise.
Our research shows that active regeneration of soliton pulses using a phase modulator can compensate temporal jitter correctly but cannot satisfactorily correct the amplitude fluctuations of the soliton pulses or prevent the accumulation of dispersive waves.
Another solution to compensating, or correcting, fluctuations of soliton pulses on an optical transmission line consists in using so-called sliding-frequency filtering, i.e. a set of filters distributed all along the line and the central wavelength of which varies from the beginning to the end of the line; for example, the central wavelength of the filter at the start of the line is the lowest and the wavelength of the filter at the end of the line is the highest. The use of such filters is described in the article by Linn F. Mollenauer xe2x80x9cSoliton transmission speeds greatly multiplied by sliding-frequency guiding filtersxe2x80x9d in Optics and Photonics News, April 1994, pages 15-19.
This xe2x80x9csliding-frequency filteringxe2x80x9d provides partial correction of the temporal jitter of the pulses without the noise increasing with the number of filters, as with fixed filtering.
Nevertheless, it has been found that using such filters is not entirely satisfactory in terms of correcting temporal fluctuations.
The invention is directed to enabling the implementation of an optical telecommunication system using pulses in the soliton format in which the correction of the fluctuations of the pulses in terms of amplitude and temporal position is particularly effective, with the aim of enabling irreproachable transmission over long distances and at a very high bit rate.
The invention starts from the observed limitations of amplitude correction, mentioned above, and of phase modulation, possibly combined with fixed filtering or dispersion compensation.
To correct the fluctuations of the soliton pulses, the invention uses the combination of sliding-frequency filtering and phase modulation. Alternatively, saturable absorption components are employed in place of the sliding-frequency filters. For information on these saturable absorption components, reference may be had, for example, to the article entitled xe2x80x9cIncreased Amplifiers spacing in a Soliton System with Quantum-Well saturable absorbers and Spectral Filteringxe2x80x9d published in Optics Letters, volume 19, number 19, Oct. 1, 1994, pages 1514-1516.
Note that in the article by N. J. Smith et al published in IEEE Photonics Technology Letters cited above it is suggested that the use of sliding-frequency filtering in combination with phase modulation is not feasible.
The maximum number of phase modulators on the line is preferably three. Nevertheless, the use of a single phase modulator, preferably half-way along the transmission line, is sufficient to achieve the correction of fluctuations in amplitude and in time of soliton format pulses.
It has been found that, using the invention, it is also possible to reduce the number of repeaters on the line, all other things being equal.
Transoceanic transmission systems having lengths between 6,000 kilometers and 10,000 kilometers using a data bit rate of 20 Gbit/s and a quality factor (of the soliton pulses) Q=12 have been compared. If sliding-frequency filtering is used on its own, it is necessary to have a distance of about 50 kilometers between two successive repeaters, a xe2x80x9csliding-frequencyxe2x80x9d filter being associated with each repeater.
If phase modulation is used in association with fixed frequency filters, the distance between successive repeaters to achieve the same results in terms of bit rate and quality factor is about 80 kilometers.
In contrast, using the invention, if a single phase modulator is used half-way along the line the minimum distance between two repeaters is 100 kilometers, still with the same bit rate of 20 Gbit/s and quality factor of 12. In this example a filter (of the sliding-frequency filtering process) is also associated with each repeater.
Other features and advantages of the invention will emerge from the description of some embodiments of the invention given with reference to the accompanying drawings.